Vapor phase cracking



Jan. 9, 1940. w. T. HANCOCK VAPOR PHASE CRACKING Filed June 28, 1938 As illustrative, section 21a may contain 58 tubes 16 feet in length, and section 2lb, including the lower vertical rows of tubes 22 and 23 extending toward the bottom of the cracking chamber, may contain 96 tubes of corresponding length. As indicated by the arrows, the hydrocarbons flow downwardly through section 2 la, counter-current I to the rising combustion gases, and then pass downwardly through tubes 23 and upwardly through tubes 22 and the remainingtubes in the narrow bank above. Y

A suitable cracking stock, for example a gas oil fraction taken from line 20a or an independent source, may be delivered through pipe 24 -into a heating coil 25 in thebottom of the cracking chamber l I. In iiowingV through coil 25, the oil is preheated by radiant heat in the combustion v, tube rows 2 and 28 within the top portion of thev zone to a suitable temperature which may or may not be sufficient to partially vaporize the oil. The vapors then flow through line 2li to a second preheating coil consisting of series: connected cracking chamber. After being further superheated in tubes 21 Aand 28, the hydrocarbons flow through line 29 into the to'p row oi tubes in the wide tube bank 2la. At this point the oil -temperature will approach vaporizing temperature, and if desired, the still may be designed and operated to give partial or complete vaporization by thev time the oil enters the wide tube bank. In then ilowing downwardly through the bank, the

. vapors are gradually heated to proper cracking temperature which, for gas oil charging stock, may be around 1000 F. to 1100 F. The oil will have become completely vaporized at least by the time it reaches the lower two or three -rows of tubes in the wide bank, and the vapors then are heated to cracking temperature inthe lower tubes exposed to the radiant heat of the combustion zone. i

- The vapors then are maintained at cracking.

temperature during their comparatively short `period of iiow through the narrow tubevbank 2lb, during which time the vapors undergo reformation to the extent of substantially reducing fixed gas losses that otherwise would occur in the absence of the soaking treatment. Inoperation, it has been observed thatthevapor temperature may drop, say F., during passage of the vapors through the narrow tube bank. However, the vapor temperature remains within the cracking temperature range, at which effective polymerization occurs. Leaving the narrow tube bank 2lb, the vapors flow through a row of tubes 29 between tube Il and section 2Ia, into discharge line 30 which may lead to anysuitable apparatus within which the cracked vapors are subjected to condensation and treatment for the recovery of desired fractions.

Ordinarily, the vapors will be maintained under low pressure, say below 25 lbs. per sq. in. gage, in the crackingcoils, Valthough any suitable pressure may be held in the coils by aback pressure valve 3| in the discharge line 30. It has been found from actual operation that not only does the system give highly satisfactory yields as compared with other vapor phase cracking processes, but also the conditions obtaining in the cracking coils are such that no excessive deposits of carbon occur inthe tubes. Accordingly, it is possible to operate the still over long periods of time without having to shut down for carbon removal.

I claim:

l. The method of cracking hydrocarbons within a heating zone formed by horizontally spaced walls, that includes `discharging incandescent` combustion gases into the lower portion oi the heating zone and conducting the gases upwardly within said zone, passing hydrocarbons downwardly and counter-current to the rising combustion gases in an extended path of iiow through a bank a series-connected, horizontally extending cracking tubes of relatively great total length and exposed within said zone to radiant heat of the combustion gases, thereby heating the hydrocarbons over `a relatively longl period of time at a rate suiciently gradual to minimize fixed gas and carbon formation and at a temperature such that the hydrocarbons are cracked in vapor phase, then passing the cracked vapors upwardly through a reduced length path of'flow in a bank of series-connected, horizontally extending soaking tubes having substantially less total length than saidcracking tubes and located within said zone in the path of the upwardly owing combustion gases directly between said cracking tubes and the wall of the heating zone, and reforming the vapors by maintaining them at cracking temperature during' their passage through said soaking tubes vand over a relatively short period of time.

in a heating zone formed by horizontally spaced walls, that includes discharging incandescent combustion gases into the lower portion of the heating zonefand conducting the gases upwardly within said zone, passing hydrocarbons downwardly and counter-current to the rising combustion gases in an extended path of` flow through a comparatively wide bank` oi' series-connected horizontally extending cracking tubes oi' relatively great total length and exposed Within said zone to radiant heat oi the combustion gases, thereby heating the hydrocarbons over a relatively long period of Vtime at a rate suiiiciently gradual to minimize xed'gas and carbon formation. and at a temperature such that the hydrocarbons are cracked in vapor phase, then passing the cracked vapors upwardly through a reduced length path of flow in a comparatively narrow bank of seriesconnected, horizontally extending soaking tubes having substantially less total length than said cracking tubes and located within said zone in the path of the upwardlyowing `combustion gases directly between said cracking tubes and the wall of the heating zone, and reforming the vapors by maintaining' them at cracking temperature during their passage through said soaking tubes and over a relatively short period of time.

3. The method of cracking hydrocarbons within a heating zone formed byhorizontally spaced walls, that includes discharging incandescent combustion gases into the lower portion of the heating zone and conducting the gases upwardly within said'zone, preheating a stream of hydrocarbons by said combustion gases and then passing the preheated hydrocarbons Vdownwardly and 2. The method of cracking hydrocarbons with-- counter-current .to the rising combustion gases in an extended path of flow through a bank` of series-connected horizontally extending cracking tubes of relatively great totalA length and exposed Within said zone to radiant heat of the combustion gases, thereby heating the hydrocarbons overay relatively long periodl of time at arate sulicient?` reduced length path of flow in a bank of seriesconnected, horizontally vextending soaking tubes path of the upwardly owing combustion gases directly between said cracking tubes and the wall of the heating zone, and reforming the vapors by maintaining them at cracking `temperature during their passage through said soaking tubes and over a relatively short period of time.

.4. The method of cracking hydrocarbons Withinv withinsaid zone, passing a stream of hydrocar flow through a bank of series-connected horilbons ythrough a preheating space in said heating zone andvthen passing the preheated hydrocar- .bons downwardly and counter-.current kto the lrising combustion gases in an extended pathof zontallyy extending cracking tubes of relatively. A

great total lengthfand exposed Within said vzone to radiant heat of the combustion gases, thereby heatingthe hydrocarbons over a relatively long' yperiod of time at arate suiciently gradual to minimize viiXed gas and carbon formation and I cracked in vapor phase, thenI passing the cracked `vapors upwardly through a reduced length path of ilow, in'abank of series-connected, horizonvtally extending soaking tubes having substan-` tially less total length than saidcracking tubes and located Within said zoney inthe path of the vupwardly owing combustion; ygases directly beata temperature such that the hydrocarbons `are having substantially lesstotal length than said cracking tubes and located Within said zone inthe l tween said cracking tubes and theWall of the heating' zone, and. reforming the vapors by maintaining thenr at 'cracking temperature during their passage* through said soaking tubes and over a relativelyA short period ottime.

fr. HANCOCK. 

